There is a number engraved on virtually every eyepiece ever made, which is the magnification, such as 10x. Futhermore, the field number (diameter of the field stop in mm) is sometimes listed, such as 10x/20. Nikon eyepieces have barrel (or sleeve) diameters of 23.2 mm or 30 mm.
The types of correction is given on many eyepieces, as well, such as CFI or CFWN. CF (or K) is "chrome-free", which means that it is corrected for chromatic aberration. W is wide-field, and H signifies "high eyepoint", or a large eye relief.
The Model S uses HKW eyepieces, which, while older, are very nice.
The SM-5 Stereo Zoom Microscope has a 30 mm diameter sleeve. Shown here is a 20x pair, with hard plastic eyecups, with almost no eye relief - you have to get your eyes right up next to the lenses. You may also notice the small physical diameter of the final lens.
The Eclipse series uses CFI eyepieces, to match the CFI objectives. While the CF system is optimized to work together, as is the CFI system, the eyepieces can be interchanged, if the sleeves fit. Shown is a pair with 30 mm sleeve diameter.
The Labophot, Optiphot and Diaphot series use the CF standard, corresponding with the objectives. They have a 23.2 mm sleeve diameter. As shown here, they have a rubber eyecup that can be folded down if desired.
This pair has the eyecups missing (which can be replaced), as well as a cracked plastic ring. They are still usable.
Here is a set of CFW 15x, which have 50% more magnification than 10x.
The CFWN 10x, shown with the eyecups out as well as folded down, are very common. Here the field number is explicitly listed as 20 mm. The "N" signifies high eyepoint, I beleive... not sure why they changed from "H".
for navigation!